The invention relates to a circuit device for supplying an alternating current of frequency f to a lamp, which circuit device is provided with a DC-AC converter comprising
input terminals for connecting the circuit device to a supply voltage source supplying a DC voltage,
a first branch including a series arrangement of a first switching element and a second switching element,
a control circuit coupled to respective control electrodes of the switching elements for rendering the switching elements conducting and non-conducting,
a load branch shunting one of the switching elements and provided with a series arrangement of an inductive element and terminals for accommodating the lamp.
Such a circuit device is disclosed in EP 0323676. In such a circuit device, the power consumed by the lamp can be adjusted, for example, by adjusting the frequency f of the control signal. A drawback of this way of adjusting the power consumed by the lamp resides in that the connection between the frequency of the control signal and the power consumed by the lamp is not unambiguous throughout the range of power consumed by the lamp. Particularly in the case of a comparatively low power consumption by the lamp, this may give rise to instabilities in the lamp operation. Another possibility of adjusting the power consumed by the lamp is to adjust the periods during which the switching elements are conducting in each period of the control signal, while the frequency of the control signal remains constant. This can be carried out symmetrically, which means that each one of the switching elements is conducting during an equal period of time in each period of the control signal. However, this can also be carried out asymmetrically, which means that the time interval during which the first switching element is conducting is unequal, in each period of the control signal, to the time interval during which the second switching element is conducting. In addition, a distinction can be made between a situation wherein one of the switching elements is conducting at any instant in a period of the control signal and a situation wherein there are time intervals during which neither switching element is conducting. In practice it has been found that asymmetrically driving the switching elements gives rise, for certain unpredictable values of power consumed by the lamp, to instabilities in the lamp. If the switching elements are symmetrically driven, a reduction of the duration during which each of the switching elements is conducting in a period of the control signal means that, during each period of the control signal, there are time intervals wherein both switching elements are non-conducting. It has been found that this way of driving the switching elements also gives rise to instabilities in the lamp, however, the values of power consumed by the lamp are predictable.
It is an object of the invention to provide a circuit device by means of which the power consumed by the lamp can be adjusted in a comparatively large range without instabilities developing in the lamp.
To achieve this, a circuit device as mentioned in the opening paragraph is characterized in accordance with the invention in that the control circuit generates a control signal at a frequency f during operation of the lamp,
for successively rendering the first switching element conducting during a first time interval and the second element during a second time interval in each first half period of the control signal, each one of the switching elements being non-conducting during the remaining part of the first half period of the control signal, and
for successively rendering the second switching element conducting during a third time interval and the first switching element during a fourth time interval in each second half period of the control signal, each one of the switching elements being non-conducting during the remaining part of the second half period of the control signal, and
in that the control signal is additionally provided with a dimming circuit for setting the duration of at least one of the two time intervals during each half period of the control signal wherein one of the switching elements is conducting. It is possible, for example, that the dimming circuit is provided with means for setting the durations of the first and the third time interval. In a preferred embodiment of a circuit device in accordance with the invention, the dimming circuit is provided with means for setting the durations of the second and the fourth time interval. It has been found that this preferred embodiment enables the power of the lamp to be adjusted in a very large range without instabilities.
During operation of a circuit device in accordance with the invention, the control signal renders the switching elements conducting and non-conducting. As a result, an alternating current of frequency f flows in the load branch. If, for example, the durations of the second and the fourth time interval can be set by means of the dimming circuit, and are set such that the difference between the durations of the first and the second time interval is maximal and the difference between the durations of the third and the fourth time interval are also maximal, then the power consumed by the lamp is maximal. At settings of the durations of the second and the fourth time interval at which these differences are smaller, also the power consumed by the lamp is smaller. It is thus possible, for example by setting the duration of the second and the fourth time interval, to adjust the power consumption by the lamp and hence the luminous flux of the lamp in a comparatively large range. It has been found that, within this range, no instabilities occur in the lamp.
Satisfactory results have been achieved with embodiments of a circuit device in accordance with the invention, wherein the duration of the second time interval is equal to the duration of the fourth time interval.
Satisfactory results have also been achieved with embodiments of a circuit device in accordance with the invention, wherein the duration of the first time interval is equal to the duration of the third time interval.
A circuit device in accordance with the invention can be embodied such that the second and the fourth time interval can be adjusted in a range from 0 to xc2xdTxe2x88x92xcex94t1 and in a range from 0 to xc2xdTxe2x88x92xcex94t3, respectively, wherein T is the duration of a period of the control signal, and xcex94t1 and xcex94t3 are the durations of the first and the third time interval. In this case, the power consumed by the lamp increases as the durations of the second and the fourth time interval increase.